1. Field of the Invention
The present invention relates to a ceramic-metal composite unit or assembly such as a ceramic turbocharger rotor, ceramic gas-turbine rotor, ceramic heating element, ceramic sensor, or the like which is used at a high temperature.
2. Disclosure Information
In recent years, ceramics have increasingly been used in various fields by the excellence of heat resistance, strength against thermal shock, mechanical strength at a high temperature, wear resistance, insulation, etc. However, the ceramics are not used alone but together with other materials such as metal so that they can show their excellent qualities.
An investigation has hereinbefore been made on how to unite a ceramic member and a metallic member with a view to obtaining a single unit or assembly having a high joint strength. This is because when the ceramic member and the metallic member are simply brazed together to constitute a single unit residual stresses are caused at the ceramic-metal joint portion due to the difference in thermal expansion efficiency between the ceramic material and the metallic material, thus causing decrease of the joint strength and therefore making it impossible to attain a sufficiently large joint strength.
This problem is pronounced when such a composite assembly is used at a high temperature.
To solve such a problem, it has been proposed to join a ceramic member and a metallic member by press fitting as disclosed in Japanese Patent Provisional Publication No. 62-191478.
To the same end, it has also been proposed to fit a ceramic member in a metallic member and fill the clearance between them by silver solder for thereby joining them together as disclosed in Japanese Patent Provisional Publication No. 2-149477.
In either of such prior art structures, the metallic member is made of an alloy such as Incoloy 903, having a low thermal expansion efficiency and consisting of Fe--Ni--Co alloy and precipitation hardening elements as Ti, Nb, Al, etc. However, by the use of such an alloy only, a favorable result cannot be attained. This is because there is a considerable difference in thermal expansion efficiency between the ceramic member and the metallic member, that is, the thermal expansion efficiency of Incoloy 903 used for forming the metallic member is for example 2.1 (.times.10.sup.-6 /.degree. C.) at 30.about.400.degree. C., whilst the thermal expansion efficiency of silicon nitride used for forming the ceramic portion is 8.2 (.times.10.sup.-6 /.degree. C.) at 30.about.400.degree. C.
As a result, when either of such prior art composite assemblies is used at a high temperature, the metallic member is expanded more than the ceramic member to make it impossible to retain a sufficient interference therebetween, thus causing a problem that the joint strength is lowered and in some case the ceramic member can be dropped off from the metallic member.